The FAA requires that fuel tanks in new airplane designs have an average flammability exposure which meets established criteria. By design, conventional unheated aluminum wing tanks can satisfy the flammability exposure criteria.
The federal regulations also require that for any fuel tank other than a main tank located in whole or in part in the fuselage (any portion of the tank being located within the fuselage contour) flammability exposure must meet additional criteria, including provisions for warm day operations.
These additional criteria do not apply to tanks designated as “main tanks.” Consequently, one means of compliance for tanks located in whole or in part with the fuselage contour is to establish that they are “main tanks,” which continuously hold “fuel reserves” necessary for continued flight and which feed fuel directly to one or more engines.
In addition to the complexities of the fuel system noted above, the implementation and functional characteristics of an aircraft fuel system and particularly the fuel tanks play a critical role in the certification and operational aspects of aircraft. The impact of fuel system design on aircraft operational capabilities encompasses a range of technologies that are more significant than most casual observers would at first realize, particularly when considering the complexities of modern jet aircraft.
Also, since fuel tanks are located in the wings, the effect of wing sweep is to change the longitudinal center of gravity (CG) of the aircraft as fuel is consumed causing a change in aircraft static stability and hence handling characteristics. Optimizing the aircraft longitudinal CG during cruise minimizes profile drag which, in turn, maximizes the operating range of the aircraft and reduces fuel consumption.
For the foregoing reasons, there is a need for a fuel tank system that permits consumption of main tank fuel first, except for a predetermined level of reserve fuel.
For the foregoing reasons, there is a need for a fuel tank that permits a reduced center of gravity envelope for the aircraft and particularly for swept wing configurations.
For the foregoing reasons, there is a need for a method to address the flammability requirements of an aircraft fuel tank that is located in whole or in part in the fuselage contour
For the foregoing reasons, there is a need for a fuel tank system that facilitates a reduction of horizontal trim loading.
For the foregoing reasons, there is a need for a fuel tank configuration that permits an optimization of structural weight.
For the foregoing reasons, there is a need for a fuel tank configuration whose benefits result in a corresponding reduction in the total quantity of fuel required to perform a given mission.
For the foregoing reasons, there is a need for establishing the inboard tanks as “main tanks” to demonstrate compliance with federal requirements for flammability criteria without the need of a flammability reduction means.